Observation of t¯t Production in Pb+Pb Collisions at √sNN=5.02 TeV with the ATLAS Detector

Abstract

Top-quark pair production is observed in lead-lead (Pb þ Pb) collisions at ffisffiffiNffiffiNffiffiffip1/4 5.02 TeV at theLarge Hadron Collider with the ATLAS detector. The data sample was recorded in 2015 and 2018,amounting to an integrated luminosity of 1.9 nb-1. Events with exactly one electron and one muon and atleast two jets are selected. Top-quark pair production is measured with an observed (expected) significanceof 5.0 (4.1) standard deviations. The measured top-quark pair production cross section isσt-t 1/4 3.6þ1.0-0.9ðstatÞ þ0.8-0.5ðsystÞ μb, with a total relative uncertainty of 31%, and is consistent with theoreticalpredictions using a range of different nuclear parton distribution functions. The observation of this processconsolidates the evidence of the existence of all quark flavors in the preequilibrium stage of the quarkgluonplasma at very high energy densities, similar to the conditions present in the early Universe.